Denny Brown Syndrome

Denny Brown syndrome is a rare paraneoplastic sensory neuronopathy (dorsal root ganglionopathy). It was historically described by Denny Brown and is now more commonly classified as paraneoplastic sensory neuronopathy or subacute sensory neuronopathy.

It is characterised by predominantly sensory, rather than motor, dysfunction, reflecting selective, immune mediated destruction of the dorsal root ganglia. It is most classically and strongly associated with small cell lung cancer and the anti Hu (ANNA-1) paraneoplastic antibody.

It frequently presents before the underlying malignancy is diagnosed. Thorough malignancy screening is therefore essential in any patient with this pattern.

Pathophysiology

• An immune mediated process, most classically associated with the anti Hu antibody, produces selective inflammatory destruction of the dorsal root ganglia. These are the sensory neuron cell bodies just outside the spinal cord.
• The antibody targets a neuronal RNA binding protein expressed in the dorsal root ganglia and, aberrantly, by the underlying tumour, most classically small cell lung cancer.
• This explains the predominantly sensory pattern, in contrast to the length dependent sensorimotor pattern of most peripheral neuropathies.
• The paraneoplastic immune response is directed against the tumour but cross reacts with antigenically similar dorsal root ganglion neurons.
• The pattern and severity of sensory dysfunction reflect the distribution and extent of dorsal root ganglion involvement in the individual patient.

• The characteristic presentation is subacute onset, over weeks to a few months, of asymmetric, often patchy sensory disturbance affecting the limbs, face and trunk. It comprises numbness, paraesthesia and frequently significant sensory ataxia, reflecting predominant involvement of large diameter, proprioceptive sensory fibres. Neuropathic pain is often severe and disabling.
• Motor function is characteristically relatively preserved, at least early, with muscle strength generally normal or only minimally affected.
• This distinguishes it from the more common length dependent sensorimotor peripheral neuropathies, where motor involvement is typically present to some degree, though often milder than sensory involvement.
• Sensory ataxia reflects loss of proprioceptive input from the affected dorsal root ganglia and can be profoundly disabling. It produces significant gait instability and difficulty with fine coordinated movement. Instability is classically worse in darkness or with eyes closed, because visual compensation for the impaired proprioception is lost.
• The neurological syndrome frequently precedes diagnosis of the underlying malignancy, sometimes by many months. The classic tumour is small cell lung cancer, though breast cancer and other tumour types have been described with this syndrome and the relevant paraneoplastic antibodies.
• Careful and persistent malignancy screening is therefore essential even where initial investigation does not identify a tumour.
• Other paraneoplastic neurological syndromes may coexist, reflecting the broader autoimmune process against shared neuronal antigens. They include limbic encephalitis, cerebellar degeneration and autonomic dysfunction, particularly in anti Hu antibody associated disease, given the relatively widespread neuronal expression of the target antigen.

• Nerve conduction studies characteristically show reduced or absent sensory nerve action potentials with relatively preserved motor conduction. This reflects selective dorsal root ganglion and sensory neuron pathology, supports the clinical diagnosis, and distinguishes it from typical length dependent sensorimotor peripheral neuropathies.
• Paraneoplastic antibody testing includes anti Hu specifically, with a broader panel given the possibility of other relevant antibodies depending on clinical context. It supports the diagnosis and guides targeted malignancy screening, as different paraneoplastic antibodies have relatively specific tumour associations.
• Malignancy screening is essential, guided by the antibody identified and the epidemiological likelihood of relevant tumour types. Use computed tomography of the chest, given the strong small cell lung cancer association with anti Hu antibody.
• Where initial screening is negative but clinical and serological suspicion remains high, add more targeted investigation including positron emission tomography imaging.
• The syndrome frequently precedes overt malignancy, so a single negative screen does not exclude a tumour. Periodic repeat screening over subsequent months is generally recommended where initial evaluation is unrevealing.
• Cerebrospinal fluid examination may show a mild lymphocytic pleocytosis and elevated protein, supporting an inflammatory, immune mediated process.
• Magnetic resonance imaging of the spinal cord excludes alternative structural causes. Image the brain too where additional neurological features suggest broader central nervous system involvement, to assess for associated paraneoplastic encephalitic involvement.

• Identifying and treating the underlying malignancy is the most important priority, alongside the obvious oncological imperative.
• Neurological outcome is frequently limited even with successful tumour treatment, because the damage is immune mediated rather than directly tumour compressive. It is nonetheless most favourably influenced by prompt tumour identification and treatment.
• Immunotherapy, including corticosteroids, intravenous immunoglobulin or plasma exchange, may be trialled. Evidence for consistent, substantial neurological improvement is limited, as paraneoplastic neurological syndromes are generally more treatment refractory than several other autoimmune neurological conditions.
• Established, often severe dorsal root ganglion neuronal loss by the time of presentation contributes to this limited response.
• Manage the frequently severe, disabling neuropathic pain by standard principles for neuropathic pain from any cause (see central neuropathic pain). Gabapentinoids, tricyclic antidepressants and other standard agents are used according to individual response and tolerability.
• Rehabilitation input is essential, given the significant, persistent functional impact even where the malignancy is successfully treated. Physiotherapy addresses the sensory ataxia and gait instability, with proprioceptive retraining and appropriate use of visual compensation strategies and mobility aids. Occupational therapy supports the functional consequences of the sensory impairment.
• The presentation spans oncological, neurological and immunological domains. Coordinated multidisciplinary management involving oncology, neurology and, where relevant, rehabilitation medicine specialists is essential.